With the 5er, there is a receptacle on the front wall of the living area down low beside the steps going up to the front area. This receptacle is opposite the back wall of the cargo area up front. I pulled out that receptacle and there was a hole into the cargo area where the batteries are up front of that space.
So, I ran an extension out from the back of that receptacle into the cargo area and added another receptacle on the end of that wire, giving me a plug-in for the deck mount converter placed up front by the batteries.
Depending on the rig's layout, any another receptacle handy could be used to extend from.